Hey there, EY. It means it has a high ph level. It's the opposite of acidic. The scale is based on food's effect after digestion. So far this site explains it best.

http://www.angelfire.com/az/sthurston/acid_alkaline_foods_list.html

Personally, I don't stress about it too much. I probably do eat a lot of foods on the acid list like saurkraut, oatmeal, coffee, and blueberries BUT I don't worry about it because I keep my wheat and sugars low (wheat and sugar are crap for many reasons, not just ph level.) And not eating, or trying to not eat, processed food- that keeps me pretty alkaline. As does my new addiction to using Bragg's Apple Cider Vinegar as salad dressing! Now as I'm perusing the list again, I see I do eat mostly alkaline. Hope that helps you!